The present invention refers to a gear pump designed to be positioned upstream of an extrusion head to convey molten masses of polymers and or elastomers and in particular highly degradable plastic materials, such as PVC and the like.
Said pumps serve to compensate for the unevenness of flow and of the vibrations caused on the material by the extrusion screw of an extruder, so as to have a constant flow of molten material coming from the pump, to obtain a high quality finished product that complies with strict tolerances.
Pumps for non-degradable extruded plastic materials, that is for products that do not give rise to problems of degradation in the event of stagnation of the material inside the pump, are known to the art. Said types of pumps are volumetric pumps that provide control of flow speed and pressure, so as to ensure a constant product output volume.
Said known pumps generally comprise a pump body consisting of a single body, substantially parallelepiped, having an inlet duct for entry of the material and an outlet duct for discharge of the material. The inlet and outlet ducts communicate with a central seat or chamber to house a pair of gears for conveying the material from the inlet duct to the outlet duct of the pump.
The central gear seat is transversally open to the outside to allow insertion of the gears. Consequently, cylindrical supports or bearings which rotatably support the shafts of the gears are mounted in the pump body to support the gears.
Between each bearing and its seat formed in the pump body, sharp edges are formed wherein the material conveyed by the pump infiltrates. In said sharp edges, the material stagnates and undergoes combustion. As a result, the material that passes through the pump carries with it the dirt and burnt matter coming from the stagnating material.
Said drawback is accentuated by the fact that the pump gears are generally self-lubricated with the same material handled by the pump which passes through the gaps between the gear shafts and the bearings which support said shafts.
The foregoing proves a serious drawback, especially in the case of highly degradable materials, such as PVC. In fact in the finished product leaving the pump, a dark stripe can be noted due to contact with the stagnating material in the pump. Said dark stripe is unacceptable, for example, for transparent plastic materials. Furthermore, said dark stripe sets off a rapid degradation of all material in close contact therewith, with the result that, after a short time, the finished product becomes unusable.
For this reason degradable plastic materials such as PVC are extruded directly, without the use of pumps at the outlet from the extrusion screw and upstream of the extrusion head. All this to the detriment of quality and accuracy of tolerances of the finished product.
The object of the present invention is to overcome the drawbacks of the prior art, providing a pump for conveying molten masses of polymers and elastomers that is able to provide an end product of high quality that complies with narrow tolerances.
Another object of the present invention is to provide a pump for conveying molten masses of polymers and elastomers that is structurally simple, and thus cheap and easy to make.
Advantageous embodiments of the invention are apparent from the dependent claims. The pump for conveying molten masses of polymers and/or elastomers according to the invention comprises a pump body within which are formed an inlet duct for entry of the material, an outlet duct for discharge of the material and a central seat communicating with the inlet and outlet ducts and intended to house at least one gear mounted on a motorised shaft for conveying material from the inlet duct toward the outlet duct.
The main feature of the pump according to the invention is represented by the fact that the pump body consists of two parts that can be assembled together. Each part of the pump body comprises at least one through hole communicating with the central chamber. In this manner at least one gear shaft is rotatably supported directly inside the pair of opposite facing holes of the two parts of the pump body.
With this solution, use of bearings or cylindrical supports mounted in the pump body to rotatably support the gear shafts is avoided. Consequently, in the pump according to the invention sharp edges between the gear shaft supports and the pump body, in which material stagnates with consequent deterioration of the quality of the end product leaving the extrusion head and downstream of the pump, are avoided.
It is evident that the pump according to the invention can be used in particular for highly degradable polymers, such as PVC (Poly Vinyl Chloride), obtaining an end product that has good quality and narrow tolerance characteristics.